1. Field of Invention
The present invention relates to a system and a method for improving performance of a lean exhaust gas aftertreatment device and, more particularly, to controlling an amount of reductant injection to achieve optimum NOx conversion efficiency while minimizing the fuel economy penalty.
2. Background of the Invention
Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx.) produced during engine operation into harmless exhaust gasses. Vehicles equipped with diesel or lean gasoline engines offer the benefits of increased fuel economy. Such vehicles have to be equipped with lean exhaust aftertreatment devices, such as, for example, an Active Lean NOx Catalysts (ALNC) or Selective Catalytic Reduction (SCR) catalysts, which continuously reduce NOx emissions, even in an oxygen rich environment, through active injection of reductant, such as fuel (HC) or urea, into the exhaust gas entering these devices. Further, it is important to precisely control the amounts of reductant in order to achieve maximum NOx conversion efficiency.
The inventors herein have recognized that transient changes in engine operating conditions cause changes in engine feedgas NOx production. For example, NOx production usually increases during engine acceleration, and decreases during deceleration. Since the amount of reductant injection is typically calculated based on steady state engine operating conditions, these transient NOx variations result in over or under-injection of reductant and negatively impact fuel economy and emission standards.